1. Field of the Inventions
The present inventions pertain to a method of coating articles; particularly, to a process by which objects are coated with carbon or silicon carbide using laser-induced chemical vapor deposition at atmospheric pressure and ambient temperature. Additionally, the present inventions relate to objects such as optical fibers that are protected from environmental corrosion by an insulating film that is applied in an open-air environment.
2. Description of the Related Art
The process of applying a carbon film to a bare optical fiber in order to hermetically seal the fiber is constantly evolving. Generally, all types of optical fibers share the same basic structure; namely, a core with a high refractive index and a surrounding cladding region with a lower refractive index. In a typical single mode optical fiber, the cladding is approximately 125 μm in diameter, while the core is approximately from 4 to 8 μm in diameter. The core and cladding are indistinguishable to the naked eye and appear as one strand of glass 125 μm in diameter. A carbon coating, which is approximately 50 to 150 nm thick, is deposited onto the cladding. Then, polymer coatings, which are on the order of 10 μm thick, are deposited on top of the carbon coating. Optical fibers are typically forged from a pure glass material, such as fused silica or fused quartz. Then, small impurities and anomalies are introduced into the material in order to obtain the desired refractive indices. The difference between the refractive index of the core and of the surrounding region facilitates the transmission of a signal along the length of the optical fiber. In order to preserve the integrity of transmitted signals and optical fibers' mechanical properties, a protective carbon film is typically applied to the external surface of optical fibers.
The system for applying a coating, film, layer, or sheath can also be used to coat the exposed section of the spliced optical fiber. Optical fiber can only be manufactured in finite lengths. However, there are scenarios such as long distance optical communication lines in which it may become necessary to join multiple fibers in order to produce a fiber longer than can be manufactured as a single strand. Two optical fibers can be joined together by a process called fusion splicing. Fusion splicing is the controlled aligning, melting, and pushing together of optical fibers resulting in a transparent, non-reflective joint. After fusion splicing an optical fiber, a section of the optical fiber is exposed such that the glass core is unprotected from environmental corrosives.
Yamauchi, in U.S. Pat. Nos. 5,223,014 and 5,360,464, discloses a technique for reinforcing and applying a carbon coating to the joint between carbon coated fibers. Yamauchi's process discloses splicing optical fibers in sealed chamber filled with inert gas at a temperature between 700° and 1000° C. using a single laser beam passing through a lens to heat the environment. After the optical fibers have been joined, a reactant hydrocarbon gas is pumped into the chamber so that a reinforcing carbon layer is deposited on the fusion spliced part.
One of the methods by which a carbon sheath is applied to a bare optical fiber is chemical vapor deposition. Chemical vapor deposition is a chain of chemical reactions which transform the gaseous molecules of a precursor gas into a thin film, on the surface of a substrate. Traditionally, the application of a carbon coating to optical fibers by chemical vapor deposition required the use of a furnace and a heated chamber. A precursor gas with carbon as one of its components is pumped into the chamber. Then, an optical fiber is drawn into the chamber. A furnace is used to heat the gas mixture and the optical fiber inside of a chamber to a temperature sufficiently high to result in the decomposition of the precursor gas. Chemical reactions that produce the carbon coating can also result from mounting the reaction chamber on a tower right below the furnace. As the optical fiber is drawn through the reaction chamber, its residual heat from the fiber draw will provide the thermal energy necessary for deposition to occur. Consequently, a carbon film is deposited on the optical fiber's surface.
Moreover, some chemical vapor deposition reactors utilize vacuum chambers. The vacuum chamber can serve three purposes. First, a decrease in the reaction enclosure's pressure results a more uniform coating, but at much lower deposition rates. The deposition rate of a system is measured in terms of either the thickness of the layer deposited on the article to be hermetically sealed divided by the time required to deposit a layer of desired thickness or the desired mass of the solid deposit material divided by the amount of time required to deposit such mass.
Unfortunately, all thickness and mass measurements must be made after completion of the coating process. The thickness of the deposited protective layer or diameter of the deposited layer can be determined by examining a cross-section of a coated optical fiber under an electron microscope with a resolution on the order of 10 nm. Due to the difficulty of obtaining a precise thickness distribution, several thickness measurements are taken across a radial cross-section. Using the average thickness measurement and assuming a constant mass density, 2.21 g cm−3 in the case of pure carbon, the total mass of the deposited film may be calculated.
Secondly, by extracting a substantial portion of the air out of the reactor, the risks of the insulating carbon layer containing impurities or not being hermetically sealed are reduced. Thirdly, the carbon film can oxidize or the precursor gas can burn when exposed to the atmosphere at temperatures required for deposition. The minimization of impurities in the carbon film and the reduction of the film's porosity provide increased protection from corrosion of the optical fiber due to contact with hydrogen, water, and other environmental contaminants. However, the utilization of a furnace, a vacuum chamber and vacuum pump, or both makes the application system bulky, inconvenient to relocate, and expensive to operate.
For this reason, some conventional carbon coating applicators use a laser to heat an optical fiber and thermally decompose only the precursor gas that is in the vicinity of the optical fiber. While utilizing a laser as opposed to a furnace reduces the size and weight of the necessary equipment, the optical fiber section to be coated is placed in a vacuum chamber in order to prevent aerial impurities from compromising the integrity and from destroying the protective properties of the carbon film. Moreover, the prior art discloses processes of carbon coating optical fibers which are performed at pressures dramatically below atmospheric pressure due to vacuum extraction.
Alternately, some prior art also discloses pumping an inert gas along with the precursor gas into the reaction chamber after a vacuum pump has extracted the majority of the air from the chamber. However, these conventional systems, which apply a carbon coating to optical fibers using either chemical vapor deposition [CVD] or laser-induced chemical vapor deposition [LCVD] still require at least a reaction chamber and vacuum pump.
Therefore, previous chemical vapor deposition processes have required bulky and expensive reaction chambers in order to control the system's pressure, temperature, and atmospheric composition.